Why are malayalees casteist when in Kerala but outside they pretend to be more accepting

Brought up in Mumbai all my life I (f28) have been very removed from the role caste plays in our lives. I went to convent school and In the city here we have friends and family from all castes, class and religion. And no one has ever mentioned anything that made me think oh caste is still prevalent here. But cut to dating a Nair boy (m28) from kannur suddenly I’m facing the repercussions of being a Thiyyar. His family are well educated people who live in Mumbai as well and we have been dating here for over a year. I didn’t even know he was Nair - he had his dad’s surname as most South Indians do! And now since we have mentioned marriage and have informed parents I’m being subjected to conversations through him from his mom of how it would be shameful to be associated to thiyyar, how his lineage would change ( because they are matrilineal ) and how he’d have to take up my surname and how bad that would be socially. While my parents were on BM finding a suitor for me from similar economical backgrounds, majority of the suitors I got were either Nairs, Pillas, Nambiars or TamBrams. While I was open to any as long as was able to connect and build a suitable relationship I am coming to believe now that it was only because of how I look. My boyfriend often comments and compliments about how fair I look. A cousin brother from Kerala once mentioned they thought my dad had married higher caste and my sister and mother are fair skinned. Another cousin had asked if my sister took injections to get fairer because “our people are not supposed to look like that” by we I mean Thiyyars. I can’t blame his family alone because I’m slowly realising that’s it’s more or less everyone that’s secretly obsessed with caste but just hid it well.